1. Field of the Invention
The present invention relates to a liquid crystal display device, and in particular to a liquid crystal display device comprising a pair of substrates opposing each other, a liquid crystal layer interposed between the substrates, and an alignment film formed on the liquid crystal layer side of the substrates.
2. Description of the Related Art
Liquid crystal display devices comprising a pair of substrates opposing each other, and a liquid crystal layer of such as nematic liquid crystals interposed between the substrates have conventionally been widely used for numeral segment type display devices such as clocks, electronic calculators and the like. On the transparent substrate of the liquid crystal display device, an active element such as TFTs (Thin Film Transistors) is formed as a switching means for selectively driving the pixel electrodes which applies a driving voltage to the liquid crystal, and further color filter layers of red, green, and blue are placed as passive elements of a color display means. On the liquid crystal layer side of the color filter layers, an alignment film is formed so as to uniformly orient the liquid crystal.
Examples of such liquid crystal display devices, as categorized based on the twist angle of the liquid crystal, are: (a) an active type, twisted nematic (hereinafter referred to as "TN") liquid crystal display system in which nematic liquid crystal molecules are aligned so as to be twisted by 90.degree.; and (b) a multiplex type, super twisted nematic (hereinafter referred to as "STN") liquid crystal display system in which nematic liquid crystal molecules having a twist angle of 90.degree. or more are used so as to attain steep voltage-light transmittance characteristics, (the voltage being applied to the liquid crystal).
In the TN type liquid crystal display device, liquid crystal molecules, which originally have refractive index anisotropy, are aligned at a certain angle with respect to the pair of substrates, on whose surface electrodes are formed, so that the contrast of a displayed image depends on the viewing angle at which a viewer observes the liquid crystal display device. In other words, the TN type liquid crystal display device has a large viewing angle dependence. In particular, as the viewing angle becomes large with respect to a normal axis direction to the display, toward a direction where the display contrast improves (normally, the front side of the display), the black and white regions of the image suddenly reverse at a point when the viewing angle exceeds a certain value. This is well known as an "inversion phenomenon".
In order to improve such a phenomenon, a pixel dividing method (Japanese Laid-open Patent Publication No. 57-186735), a rubbing angle optimizing method (Japanese Laid-open Patent Publication No. 4-221935), a non-rubbing method (Japanese Patent Publication No. 3-14162), and the like have been proposed.
The pixel dividing method, for example, can eliminate the reverse phenomenon and viewing angle dependence along the vertical direction. However, this method has problems in that: the display contrast lowers; a black image appears grey when the viewing angle is tilted from the normal axis direction, i.e., a discoloration problem; and the liquid crystal display device has viewing angle dependence along a horizontal direction (i.e., from left to right) and/or a vertical direction (i.e., from top to bottom). The rubbing angle optimization method, although capable of improving the inversion phenomenon between white and black images, has problems in that a displayed image can appear properly in only a small range of viewing angles in a direction opposite to a direction along which an inversion phenomenon may occur, and that the display contrast when the liquid crystal display device is viewed at the normal axis direction decreases. The non-rubbing method has a problem in that disclination lines remain conspicuous.
The present inventors have examined the alignment films of a spherulite structure (i.e. crystalline structure in which the crystal is radially grown) containing a crystalline polymer for the purpose of accomplishing the wide viewing angle in all directions without performing the rubbing procedure.
The above-described alignment film of a spherulite structure has an important relationship between its spherulite diameter and its display characteristics. An alignment film having a smaller spherulite diameter provides better display characteristics such as contrast ratio, coarseness, etc., among those made from the same materials. Also, an alignment film having a rougher surface provides the smaller spherulite diameter, among those made from the same materials. However, the method which controls the spherulite diameter utilizing surface roughness is not suitable for a TFT substrate having a smooth surface. Thus, in the case where the alignment film is used in a device having TFT substrates, it is difficult to accomplish the formation of smaller spherulites in the alignment film, which causes some problems in the display characteristics such as contrast ratio, coarseness, etc.